The use of various compounds as markers or taggants for liquid and solid materials is well known. Fluorescent dyes have been used in many applications, the fluorescence characteristics of a sample of the marked material being used to determine the presence and concentration of the taggant in the material. Other known taggants include biological compounds, especially DNA and oligonucleotides, and also halogenated chemicals such as perfluorocarbons. A typical application of these taggants is in the tagging of liquids such as hydrocarbon fuels in order to identify the liquid at a subsequent point in the supply chain. This may be done for operational reasons, e.g. to assist in distinguishing one grade of fuel from another, or for other reasons, in particular to ensure fuel quality, deter and detect adulteration and to provide a means to check that the correct tax has been paid. Apart from fuels, other products, such as vegetable oils may be marked to identify the product produced at a particular source, which may be licensed to produce or certified to a particular standard.
A problem with the method of detecting fluorescent compounds used as markers arises when the material which is marked interferes with the fluorescence of the marker, by absorbing the excitation or emitted light, by exhibiting its own background fluorescence, or by changing the fluorescent characteristics of the marker. This is a particular problem in the marking of coloured liquids such as petroleum derived products with fluorescent dyes because hydrocarbon based liquids, such as fuels, exhibit a broad fluorescent emission. The fluorescent background tends to add to any fluorescent signal of the dye but the inherent absorbance of the liquid diminishes the fluorescence of the dye. The marking of such fuels, especially gasoline and diesel, is an important use of marker compounds and the ability to detect single or multiple marker compounds with a high degree of certainty is critical to the use of such markers in such valuable and widespread products. The problem has been addressed in many ways, most of which involve the separation of the marker compound from the liquid by means of extraction into a polar liquid or onto a solid absorbent. For example, U.S. Pat. No. 5,358,873 describes and claims a method of detecting gasoline adulteration by tagging with a rhodamine dye and then shaking a small sample of the suspected fuel in a vial containing a small quantity of un-bonded flash chromatography-grade silica. The presence of the rhodamine marker dye in the suspect sample colours the silica red. U.S. Pat. No. 4,659,676 describes a fluorescently labelled complex hydrophobic fluid produced by dissolving therein a porphyrin. The fluorescently labelled complex hydrophobic fluid is identified by observation of the characteristic fluorescence upon irradiation. For identification purposes the porphyrin may be first extracted into an acidic aqueous solution for determination of fluorescence. U.S. Pat. No. 2,392,620 describes the use of umbelliferone or a derivative as a fluorescent marker for petroleum with detection by determination of the characteristic fluorescence after extraction into an aqueous alkaline solution. In U.S. Pat. No. 4,735,631, fuels are marked with certain substituted anthraquinones which are subsequently detected in a marked sample of fuel by extraction into an immiscible alkaline reagent.
Although DNA has been described for use as a taggant for hydrocarbon fuels, the quantitative detection of nucleic acids, for example using hybridisation or quantitative PCR methods, is not sufficiently reproducible to encourage its use as a marker for fuels, where detection of dilution or adulteration of the fuel by detection of relatively small differences in the concentration of the taggant is required.
WO2008/019161 describes a method of fuel identification with surface enhanced Raman spectroscopy (SERS) tags. This method includes the association of a substance having a known Raman spectrum with a quantity of fuel. In one embodiment, a nanoparticle including a SERS active core may be mixed into a fuel supply. In an alternative embodiment, a SERS active dye including a Raman active reporter molecule may be mixed with a quantity of fuel. If the quantity of fuel is tagged with a dye having Raman active reporter molecules, the process of identifying the quantity of fuel may include mixing into a sample of the fuel a colloid of Raman enhancing metal particles and then acquiring the Raman spectrum of the Raman active reporter molecule associated with the tag. Suitable metals include, but are not limited to, silver or gold. Alternatively, a portion of the sample may be associated with a SERS active substrate. Although a semi-quantitative example of the procedure is described in WO2008/019161, we have found that the SERS response of the tags tend to vary such that the results include a significant uncertainty due to non-reproducibility.